CLAY  SEAMS  OR  SO-CALLED  HORSEBACKS  NEAR 
SPRINGFIELD,  ILLINOIS 


T.  E.  SAVAGE 


Px'Onomic  Geology  Publishing  Company 


[Reprinted  from  Economic  Geology,  Vol.  V.,  No.  2,  March,  1910.] 


CLAY  SEAMS  OR  SO-CALLED  HORSEBACKS  NEAR 
SPRINGFIELD,  ILLINOIS. 

T.  E.  Savage.1 

Clay  filled  fissures  in  the  form  of  clay  seams  are  not  uncommon 
in  the  Upper  Carboniferous  coal  beds  of  the  United  States. 
They  have  been  described  from  Pennsylvania,  West  Virginia, 
Ohio,  Indiana,  Illinois,  Iowa,  Missouri  and  Kansas.  In  some 
cases  the  clay  appears  to  have  been  derived  from  the  fire  clay 
below  the  coal,  and  in  others  its  source  seems  to  have  been  some 
horizon  above  the  coal. 

Describing  clay  seams  in  the  Kansas  coal  beds,  Crane2  says : 

“ There  is  always  an  upward  displacement  of  shale  at  the  upper  ex- 
tremities of  the  clay  veins  ...  in  all  cases  the  clay  in  the  clay  veins  is 
similar  to  that  underlying  the  coal;  and  . . . the  horizontal  position 
of  the  coal  or  shale  strata  has  not  usually  been  disturbed.” 

Concerning  their  probable  origin  he  states : 

“ Long  after  the  coal  was  formed  and  consolidated  almost  to  its  pres- 
ent condition,  vibratory  movements  of  some  kind  fissured  the  strata  in- 
cluding the  coal  beds.  After  the  fissures  were  formed  the  fire  clay 
below  the  coal  was  squeezed  upward,  filling  the  fissure  by  the  process  of 
ordinary  creeping.  The  upturning  of  the  shale  laminae  near  the  upper 
part  of  the  fissure  would  be  readily  produced  by  the  upward  movement 
of  the  clay  under  the  power  which  forced  it  along.” 

His  Fig.  23  on  page  203  shows  a clay  seam  in  which  the  down 
bending  of  the  edges  of  shale  and  coal  indicate  that  the  movement 
of  the  cla)r  was  downward  in  the  fissure,  while  Fig.  29  on  page 
209  represents  a clay  seam  in  which  the  clay  appears  to  have 
moved  up  from  below  the  coal. 

The  coal  field  about  Wier  City  is  mentioned  as  showing  very 
numerous  clay  seams.  On  page  164  is  described  coal  on  the 
Kepple  farm  in  that  vicinity,  four  feet  thick,  with  a floor  of  good 
fire  clay,  having  a roof  composed  of  six  feet  of  sandy  shale  above 

1 Illinois  Geological  Survey. 

2 Crane,  Univ.  Geol.  Surv.  Kan.,  Vol.  III.,  pp.  21 1 and  212. 


79 


T.  E.  SAVAGE. 


which  occurs  two  feet  of  fire  clay,  which  is  succeeded  by  more 
sand  and  shale.  The  above  conditions  of  roof  and  floor  would 
seem  to  be  favorable  for  the  development  of  clay  seams  on  the 
same  principle  as  those  occurring  in  the  Springfield  area. 

Keyes1  states  that: 

“ Ruptures,  or  more  or  less  vertical,  simple  fissures,  which  are  merely 
a separation  of  different  parts  of  the  coal  seams  without  displacement, 
are  not  uncommon  in  Iowa.  Clay  usually  fills  the  fissure  which  may 
be  from  an  inch  to  a foot  or  more  in  width.” 

From  the  figures  which  he  gives  of  these  clay  seams,  the  direc- 
tion of  movement  of  the  clay  in  the  fissures  cannot  be  determined. 

Bain2  has  figured  a clay  seam  in  which  the  down  bending  of 
the  edges  of  coal  and  roof  slate  at  the  top  of  the  coal  seam  indi- 
cate the  downward  movement  of  the  clay  in  the  fissure,  similar 
to  those  in  the  Springfield  area  where  the  fissure  passes  nearly 
vertically  into  the  coal.  The  roof  of  the  coal  in  this  region  con- 
sisted of  one  and  one  third  feet  of  black,  fissile  shale,  succeeded 
by  about  four  feet  of  limestone  above  which  were  seven  feet  of 
pyritic  shale. 

Discussing  clay  veins  in  the  Indiana  coal  seams  Ashley3  says : 

“ They  are  usually  associated  with  faults  and  are  probably  due  to  the 
action  of  the  same  forces.  ...  In  a few  places  the  clay  appears  simply 
to  fill  the  space  made  by  the  separation  of  the  rocks  on  either  side  of 
the  line  of  faulting.  In  most  cases  the  coal  and  the  rock  on  either 
side  seem  to  have  been  broken  up  . . . and  when  the  clay  was  forced 
up  through  this  mass,  it  extended  out  into  it,  following  lines  of  least 
resistance,  and  catching  up  masses  of  it  in  the  main  clay  body.” 

In  his  sketches  of  clay  seams,  Figs.  3 and  4 on  page  58  of  this 
report,  there  seems  to  be  indicated  a down  bending  of  the  upper 
edge  of  the  coal  on  the  overhanging  side  of  the  fissure  without 
any  real  faulting  of  the  bed.  On  page  468  he  describes  a coal 
seam  on  the  Conce  land,  four  and  one  half  feet  thick,  which  is 
overlain  by  twenty-two  inches  of  black  shale,  above  which  are 
three  feet  of  clay  shale.  Of  another  mine,  nearby,  the  fire  clay 

1 Keyes,  Iowa  Geol.  Surv.,  Vol.  II.,  p.  189. 

2 Bain,  Iowa  Geol.  Surv.,  Vol.  V.,  p.  399. 

3 Ashley,  23d  Ann.  Rept.  Dept.  Geol.  and  Nat.  Res.  of  Indiana,  p.  59. 


CLAY -SEAMS  NEAR  SPRINGFIELD , ILLINOIS.  180 

is  said  to  give  no  trouble  creeping,  possibly  because  too  dry.  Clay 
seams  are  said  to  be  common  in  the  coal  in  this  vicinity. 

The  conditions  here  of  a dry  fire  clay  floor;  a hard,  black  shale 
roof,  followed  by  a three-foot  bed  of  clay  shale  are  consistent 
with  the  state  of  things  in  the  No.  5 seam  of  the  Springfield  area. 

Concerning  the  clay  seams  in  Ohio  coal  beds,  Orton1  states 
that : 

“ Clay  veins,  one  of  the  most  serious  drawbacks  to  mining  in  many 
fields,  are  connected  with  and  proceed  from  the  fire  clay  floor.  They 
seem  to  have  been  formed  in  the  earlier  stages  of  the  history  of  the 
coal  seam  by  some  inequality  of  pressure  or  resistance  whereby  the 
bottom  clay  was  forced  in  thin  sheets  through  the  hardening  coal.” 

In  his  report  on  the  coals  of  Michigan  Lane2  says  that  clay- 
filled  veins,  or  clay  seams,  are  but  little  known  in  Michigan.  An 
inspection  of  the  various  shaft  sections  given  in  this  report  shows 
a remarkably  small  amount  of  fire  clay  or  soft  shales  associated 
with  the  coal  seams,  and  a great  predominance  of  sandstone  and 
hard  shale.  Such  strata  associated  with  the  coal  beds  would  not 
be  favorable  for  the  development  of  clay  seams. 

In  discussing  clay  veins,  more  especially  those  of  the  Pennsyl- 
vania coal  field,  Gresley3  thinks  that  the  clay  vein  fissures  may  be 
placed  in  the  category  of  earthquake  phenomena.  He  says : 

“ The  fissures  remained  open  long  enough  to  allow  weathering  and  the 
gradual  disintegration  of  their  ragged  sides.  The  character  and  aspect 
of  the  finer  clay  and  the  more  fragmentary  ingredients  of  the  clay  veins 
are  such  as  to  indicate  that  they  were  principally  derived  from  the 
walls  of  the  veins.  In  most  cases  the  vein  stuff  comes  by  descent  from 
the  walls,  in  other  cases  they  are  intrusively  filled  by  the  underclays 
which  have  been  squeezed  up  into  the  open  cracks  in  the  overlying  coal.” 

It  seems  probable  that  clay  seams  have  been  formed  in  different 
ways  under  differing  conditions  of  roof  and  floor,  and  varying 
degrees  and  kinds  of  strains  to  which  the  strata  were  subjected. 
It  is  not  possible  that  earthquake  phenomena  or  general  crustal 
strains,  such  as  produce  widespread  faulting,  could  be  concerned 

1 Orton,  Geol.  Surv.  of  Ohio,  Vol.  V.,  p.  143. 

2 Lane,  Geol.  Surv.  of  Mich.,  Vol.  VIII.,  p.  126. 

8 Gresley,  Bull.  Geol.  Soc.  of  Amer.,  Vol.  IX.,  p.  35,  et  seq. 


1 8 1 


T.  E.  SAVAGE . 


in  the  formation  of  the  fissures  of  the  clay  seams  in  the  Spring- 
field  region.  Fractures  from  the  above  mentioned  causes  would 
not  be  limited  to  a few  feet  in  vertical  height,  or  to  one  particular 
coal  seam,  as  in  area  under  discussion. 

Clay  Seams  Near  Springfield. — The  principal  coal  seam  ex- 
ploited in  western  Illinois,  over  the  area  between  Springfield  and 
Peoria,  was  designated  in  the  Worthen  reports  as  No.  5.  The 
following  section  shows  the  character  of  the  strata  associated  with 
the  No.  5 coal  in  the  region  about  Springfield. 


Feet. 

Sandstone  or  shale 10  to  30 

Light  gray  shale  or  soapstone i]/2  to  4 

Limestone  cap  rock y2  to  1% 

Black,  fissile  shale 2^4  to  454 

Coal  (No.  5) 5 to  6y2 

Fire  Clay il/2  to  5 


One  of  the  conspicuous  features  of  the  No.  5 coal  seam  is  the 
presence  of  numerous  clay-filled  fissures  that  extend  down  into, 
or  through,  the  coal  seam  from  the  overlying  beds.  The  fissures 
are  generally  from  two  or  three  to  sixteen  inches  in  width, 
although  the  larger  ones  attain  a width  of  three  or  four  feet. 
Their  walls  are  slickensided,  but  do  not  show  any  traces  of  weath- 
ering. The  spaces  between  the  walls  are  filled  with  a light  gray 
shale  or  soapstone. 

These  fissures,  with  their  fillings,  are  known  to  the  miners  as 
horsebacks.  There  is  no  regularity  in  the  distance  between  these 
horsebacks,  or  clay  seams,  or  in  the  direction  in  which  they  ex- 
tend. In  some  mines  they  are  encountered  about  forty  to  sixty 
feet  apart,  while  in  others  they  are  separated  by  a distance  of  two 
hundred  to  four  hundred  feet,  or  more.  They  traverse  the  coal 
seam  in  various  directions,  no  single  direction  greatly  predomi- 
nating even  in  the  same  mine. 

The  shale  filling  the  fissures  is  light  gray  in  color  and  is  gener- 
ally rather  soft.  In  rare  cases  it  is  so  hard  that  it  emits  sparks 
when  struck  with  a hammer,  but  usually  it  soon  slakes  down  into 
an  incoherent  mass  upon  exposure  to  the  air.  Where  the  horse- 
back enters  the  top  of  the  coal  seam  the  fissure  at  once  becomes 


CLAY-SEAMS  NEAR  SPRINGFIELD,  ILLINOIS. 


182 


wider.  The  upper  laminae  of  the  coal,  immediately  adjacent  to 
the  fissure  on  the  overhanging  side,  are  more  or  less  steeply  bent 
downward;  the  bending  or  buckling  of  the  layers  fading  out 
laterally  within  a few  feet  from  the  fissures.  Fragments  of  the 
black  shale,  from  the  roof  of  the  coal,  were  seen  at  many  points 
in  the  clay  filling  of  the  horsebacks  from  five  to  twenty-nine 
inches  below  the  top  of  the  coal.  In  mine  No.  5 of  the  Spring- 
field  Coal  Mining  Company,  a fragment  of  coal,  six  inches  long 
and  three  fourths  of  an  inch  in  thickness  was  found  in  the  clay 
of  a horseback  nine  inches  below  the  bottom  of  the  coal  seam. 


SAND  STONE 
GRAY  SOAPSTONE 
CAP  ROCK 


BLACK  SHALE 


COAL  NO.  5 


FIRE  CLAY 


Fig.  21.  Typical  Clay  seam  or  horseback  in  Mine  No.  5 of  the  Springfield 

Coal  Mining  Co. 

In  the  mine  last  mentioned  there  were  seen  in  three  of  the 
horsebacks  a slight  upward  bending  of  the  lower  edge  of  the  coal 
on  the  side  of  the  fissure  opposite  to  that  in  which  the  down  bend- 
ing at  the  top  occurred.  This  upward  bending  at  the  bottom, 
however,  is  only  one  third  to  one  half  as  great  as  the  down  bend- 
ing of  the  coal  at  the  top  of  the  seam  in  the  same  horseback. 
When  the  clay  seam  passes  into  the  coal  bed  in  a nearly  vertical 


83 


T.  E.  SAVAGE. 


direction  there  is  a down  bending  of  the  coal  at  the  top  of  the 
seam  on  each  side  of  the  fissure.  However,  the  more  nearly 
vertical  the  direction  in  which  the  horseback  cuts  through  the  coal, 
the  less  is  the  distance  through  which  the  edges  at  the  top  and 
bottom  of  the  seam  are  bent. 

In  no  instance  was  there  seen  a true  faulting  of  the  beds.  I 
wish  to  emphasize  the  fact  that  in  no  case  was  there  a slipping  of 
the  middle  part  of  the  coal  seam  on  one  side  of  the  fissure  above 
the  level  of  the  corresponding  part  of  the  seam  on  the  opposite 
side.  The  only  vertical  displacement  is  a downward  pushing  of 
the  cap  rock  and  roof  shale,  and  a down  bending  of  the  upper 
laminae  of  the  coal  on  the  overhanging  side  of  the  fissure,  through 
a vertical  distance  of  from  two  to  twenty  inches;  and,  less  fre- 
quently, a much  smaller  upward  bending  of  the  lower  edge  of  the 
coal  seam  on  the  opposite  side  of  the  fissure,  as  shown  in  Fig.  21. 

In  this  area  the  fissures  have  a very  limited  vertical  extent.  In 
the  Mechanicsburg  mine  a coal  seam  was  formerly  worked  about 
thirty-five  feet  above  the  No.  5 bed  which  is  at  present  being 
mined.  Although  these  two  coal  seams  are  separated  by  an 
interval  of  less  than  forty  feet,  the  No.  5 coal  is  traversed  by 
numerous  clay  seams,  while  none  were  encountered  in  the  upper 
bed. 

It  seems  certain  that  there  was  a somewhat  ready  yielding  of 
the  coal  mass  in  a lateral  direction  when  the  fissures  were  formed. 
This  is  shown  in  the  fact  that  the  walls  of  the  fissures  are 
wider  apart  in  the  coal  bed  than  in  the  overlying  roof  shale  and 
cap  rock.  The  clay  seams  always  cut  the  bed  in  an  oblique  or 
a vertical  direction,  never  following  partings  or  stratification 
planes  of  the  coal  seam,  even  where  these  are  well  marked.  The 
amount  of  downward  slipping  of  the  cap  rock  is  always  less  than 
the  extent  to  which  the  upper  edge  of  the  coal  seam  overhanging 
the  fissure  is  bent  downward.  All  of  the  movement,  both  hori- 
zontal and  vertical,  that  has  taken  place  in  connection  with  the 
clay-filled  fissures,  seems  to  have  been  made  possible  by  the  yield- 
ing of  the  materials  within  the  coal  seam  itself. 

That  the  coal  offered  accommodation  to  the  strains  that  caused 


CLAY-SEAMS  NEAR  SPRINGFIELD,  ILLINOIS. 


184 


the  formation  of  the  fissures  is  shown  by  the  fact  that  within  the 
coal  seam  the  smaller  fissures  often  divide  into  a number  of 
branches  which  eventually  die  out*  without  passing  entirely 
through  the  bed. 

The  clay  filling  the  fissures  is  thought  to  have  been  pressed 
downward  through  the  break  in  the  cap  rock  and  roof  shale, 
into  the  coal,  from  the  bed  of  gray  shale  or  soapstone  overlying 
the  cap  rock.  As  this  clay  was  forced  downward  into  the  fis- 
sures it  caught  the  overhanging  and  unsupported  edges  of  the 
cap  rock,  roof  shale  and  coal,  bending  those  edges  downward 
on  the  overhanging  side  of  the  fissure. 

That  this  clay  was,  for  the  most  part,  squeezed  downward 
from  above  the  coal  horizon  is  indicated  by  the  manner  in  which 
the  upper  edge  of  the  coal  overhanging  the  fissure  has  been  bent 
downward  to  a much  greater  extent  than  the  lower  edge  of  the 
coal  on  the  foot-wall  side  has  been  forced  upward.  It  is  shown 
in  the  fact  that  fragments  of  black  shale  from  the  roof  of  the 
coal  are  common  in  the  clay  filling  of  the  horseback  several  inches 
below  the  top  of  the  coal  seam ; the  fragments  of  coal  also  occur 
in  the  clay  some  inches  below  the  bottom  of  the  seam.  In  no 
case  were  coal  fragments  found  in  the  clay  filling  at  a level 
higher  than  the  top  of  the  coal. 

Suggested  Origin  of  the  Local  Clay  Seams. — It  is  thought 
that  the  formation  of  the  clay-filled  fissures  was  intimately  re- 
lated to  the  character  of  the  beds  above  the  No.  5 coal.  It  is 
probable  that  the  character  of  the  underlying  fire  clay,  which 
here  is  dry  and  does  not  creep  readily,  is  also  a conditioning 
factor.  The  fissures  were  not  formed,  as  at  present,  until  after 
the  vegetal  mass  composing  the  coal  seam  had  been  compressed 
to  near  its  present  volume.  This  is  shown  in  the  fact  that  the 
clay  seams  show  no  bending  or  buckling,  such  as  would  result 
if  much  compression  of  the  coal  had  occurred  after  the  horse- 
backs were  completely  formed.  Where  the  lower  edge  of  the 
coal  rests  upon  the  clay  filling  the  horseback,  the  laminae  are  not 
curved  upward  adjacent  to  the  fissure  as  they  would  be  if  the 
coal  at  a distance  from  the  fissure  had  been  compressed  or  set- 


i«5 


T.  E.  SAVAGE. 


tied  downward  since  the  horseback  was  formed.  That  a degree 
of  consolidation  of  the  coal  sufficient  to  permit  of  jointing  had 
occurred  prior  to  the  formation  of  the  horsebacks  is  shown  by 
the  fact  that  in  some  places  the  clay  from  the  fissures  has  spread 
into  the  joints  of  the  coal  adjacent  to  the  horseback. 

Campbell1  suggests  that  joints  are  developed  early  in  the  proc- 
ess of  coal  formation,  and  that  the  carbonization  of  the  coal 
beyond  the  lignite  condition,  depends  upon  the  presence  of  joints 
and  cleavage  planes  along  which  the  gases  could  find  a way  of 
escape.  If  this  is  the  case,  there  would  be  a considerable  amount 
of  compression  and  contraction  of  the  coal  seam  after  the  joints 
were  formed,  before  the  vegetal  mass  reached  the  condition  of 
bituminous  coal. 

It  is  assumed  that  as  the  mass  of  vegetal  material,  under  the 
weight  of  overlying  sediments,  was  slowly  transformed  into 
coal,  there  would  be  somewhat  unequal  contraction  in  different 
parts  of  the  seam,  owing  to  the  lack  of  homogeneity  of  the  vege- 
tal materials  making  up  the  coal  beds;  and  that  the  contraction 
of  the  coal  materials  continued  long  after  a high  degree  of  con- 
solidation of  the  coal  had  taken  place.  So  long  as  the  materials 
possessed  some  degree  of  mobility  the  unequal  shrinking  in  the 
different  parts  of  the  coal  seam  would  be  equalized  by  the  move- 
ment of  some  of  the  mass  towards  the  points  of  least  pressure. 
When  the  consolidation  reached  a certain  point  such  adjustment 
would  be  no  longer  possible.  After  this,  the  continued  unequal 
shrinking  of  the  vegetal  mass  would  cause  unequal  strains  in  the 
roof  of  the  coal  under  its  load  of  superposed  sediments. 

If  the  roof  of  the  coal  seam  was  a soapstone,  or  somewhat 
plastic  shale,  the  mobility  of  the  shale  particles  would  permit  this 
zone  to  adjust  the  inequalities  of  strain  resulting  from  the  une- 
qual contraction  of  the  coal  seam.  Such  conditions  exist  in  the 
roof  of  coal  No.  6,  in  the  Carterville-Zeigler  district  of  southern 
Illinois.  Rock  rolls  or  depressions  in  the  top  of  the  coal  are 
here  common,  but  no  clay  seams  penetrate  the  coal  bed.  In  the 
vicinity  of  these  rolls  the  roof  shale  is  cut  by  slickensided  zones 

1 Campbell,  Economic  Geology,  Vol.  I.,  No.  i,  p.  30. 


CLAY-SEAMS  NEAR  SPRINGFIELD , ILLINOIS.  1 86 

for  a distance  of  several  feet  from  the  center  of  the  roll,  indi- 
cating a considerable  lateral  movement  in  the  shale  in  accom- 
plishing the  adjustment  of  the  strains.  However,  the  roof  of 
coal  No.  5 is  a hard,  brittle  shale  whose  constituent  particles  do 
not  possess  the  mobility  requisite  for  such  adjustment.  If  the 
limestone  cap  rock  was  very  thick  it  might  be  able  to  withstand, 
without  fracture,  the  unequal  strain  due  to  the  unequal  contrac- 
tion in  the  underlying  coal  seam.  The  cap  rock  of  this  coal  is 
thin,  averaging  only  twelve  to  fourteen  inches.  The  combined 
strength  of  the  roof  shale  and  cap  rock  was  not  sufficient  to  with- 
stand the  unequal  strain  to  which  they  were  subjected,  and  Assur- 
ing of  the  beds  resulted. 

Immediately  above  the  cap  rock  of  this  coal  seam  is  a bed  of 
rather  soft,  gray  shale  or  soapstone  whose  particles  were  suffi- 
ciently mobile  to  bring  about  adjustment  in  the  unequal  strains 
which,  by  the  Assuring  of  the  roof  shale  and  cap  rock,  had  been 
transferred  to  this  higher  horizon.  The  materials  from  this 
shale  horizon  were  immediately  squeezed  downward  through  the 
Assures  as  a wedge  into  the  coal  seam  until  the  inequality  of 
pressure  was  adjusted.  Under  these  conditions  the  place  in 
which  adjustment  was  accomplished  was  limited  to  a narrow 
zone  below  the  point  where  the  fracture  was  made  in  the  roof 
shale  and  cap  rock.  Hence  the  effects  are  conAned  to  a narrow 
zone  horizontally  but  they  became  thus  strongly  marked  in  a 
vertical  direction. 

It  is  probable  that  from  time  to  time,  as  the  shrinking  in  the 
coal  mass  continued,  more  clay  was  forced  downward  into  the 
coal  seam,  Assuring  it  still  deeper  and  spreading  the  walls  of  the 
Assure  constantly  wider  apart.  The  abundant  evidences  that 
the  clay  Ailing  the  Assures  in  the  SpringAeld  coal  seam  was 
pressed  down  from  above  the  coal  have  been  given  on  a preced- 
ing page.  In  this  manner  also  the  slickensiding  was  accom- 
plished by  the  slipping  of  the  clay  in  the  Assure,  and  not  by  the 
movement  of  the  walls  of  the  Assure  upon  each  other. 

It  is  thought  that  the  principle  of  unequal  contraction  in  the 
different  parts  of  the  coal  seam,  during  the  progress  of  its  con- 


i«7 


T.  E.  SAVAGE. 


solidation,  applies  also  in  the  formation  of  the  more  common 
types  of  rock  rolls  in  the  top  of  coal  seams.  The  character  and 
sequence  of  the  beds  above  the  coal  are  considered  the  chief  fac- 
tors in  determining  whether  rolls  or  clay  seams  will  be  formed 
in  the  adjustment  of  the  strains  arising  from  such  unequal  con- 
traction. 


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